With 485, the third wire is optional. It is recommended if the grounds at the two endpoints are not "close" The signals are voltage referenced not current referenced
Because 485 can have such long cable runs (over a kilometer) the grounds in each building might be very different, if so you need the ground reference wire. But a short cable in one room, I think two wires works as long as all the power supply share a common ground reference. I think using TTL levels works OK with many RS-232 systems but I doubt it would work well with 485. You need a "real" differential driver that can send negative volts. There is no need to guess and experiment if you have a dual channel 'scope. Look at the far end of the cable and see if symmetric voltages with nice square edges are present. On Fri, Jan 1, 2021 at 9:27 AM John Dammeyer <[email protected]> wrote: > > -----Original Message----- > > From: [email protected] [mailto:[email protected]] > > Den Fredag, Januari 01, 2021 06:03 CET, skrev Chris Albertson < > [email protected]>: > > �I don't know much about RS485 but I did read that the current return > path > > is required. You should be able to send and about 100K up to 4000 feet. > > > > RS-485 use differential signals, currents flowing in differention > directions so return path is in the other conductors. A termination is > > needed, should be easy to find how it should be made ib internet once > you know it's needed. > > > > As Dan stated RS485 changes the polarity of the signal so current flows > from Pin A to Pin B for one logic level and from Pin B to Pin A for the > other. Unlike CAN bus, a ground is _not_ needed. > > But there are several rules to follow. One of them is the impedance of > the cable should be in the neighborhood of 100 Ohms. Using 50 Ohm cable is > pushing it. > > The cable, with twisted pair is a transmission line and as such needs to > be terminated across the A,B pins with a resistor that matches the cable > impedance. So a 50 ohm cable needs a 50 Ohm resistor at each end. That's > a parallel combination of 25 Ohms load plus the 2x 12K receiver input > impedance. For now forget the input. > > Differential output from the TI SN75176 data sheet at 54 Ohms is Typically > 2.4V. So 2.4/54=44mA. > > Now use two 50 Ohm resistors and 2.4V/25 Ohms = 96mA which exceeds the > maximum drive current. Assume the low output voltage of 1.7V/25 Ohms and > you get 68 mA which is still 8mA above max. > > If you don't use 50 ohm resistors on 50 Ohm impedance cable then you get > reflections from the edges of the data signals. Again, might not be a big > deal on a short cable or even a long one. But the right combination of 1's > and zero's might well create a travelling wave that randomly distorts the > signal with no repeatability or possibility of testing. > > I suggest finding a piece of cable that is rated at 100 Ohm or 120 Ohm > impedance (twisted pair) and using that with proper resistors at each end. > > > Say you google "shielded twisted pair cables" and chose the link for the > Beldon 9451. Seems pretty good from the Digikey spec list except they > interestingly don't list the impedance. For that you need to download the > Beldon data sheet and there you find the characteristic impedance is 45 > Ohms. That' pretty typical. > > Now add 100 Ohm to the search "shielded twisted pair 100 Ohm cables" > First hit is for 9501. You have to look carefully to see it's 75 Ohm. > > There are other brands. > > John Dammeyer > > > > > _______________________________________________ > Emc-users mailing list > [email protected] > https://lists.sourceforge.net/lists/listinfo/emc-users > -- Chris Albertson Redondo Beach, California _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
